1. Field of the Invention
This invention relates generally to positron emission tomography and more particularly to devices which use an array of scintillation detectors to detect the annihilation radiation from positron disintegration and use this information to reconstruct an image of the distribution of positron emitting isotopes within a body.
2. Description of the Prior Art
Positron emission tomography is a technique for measuring the concentration of a positron emitting isotope through a sectional plane through the body. Normally the isotope is used to label a substance which circulates with the blood and which may be absorbed in certain tissues. The technique allows the actual concentration in the slice to be determined after suitable calibration of the device.
Certain isotopes decay by emitting a positively charged particle with the same mass as the electron (positron) and the neutrino from the nucleus. In this process one of the protons in the nucleus becomes a neutron, so that its atomic number goes down while its atomical weight remains constant. This positron is ejected with a kinetic energy of up to 2 Mev depending on the isotope and loses this energy by collisions while travelling a distance of up to a few mms in water. When it has reached thermal energies it interacts with an electron and they mutually annihilate one another. The rest mass of the 2 particles is transformed into 2 gamma rays of 511 Kev which are emitted at 180.degree. in the `center of mass` coordinates of the original particles. The 2 gamma rays may be detected by suitable devices. If these devices measure the energy of the gamma rays at about 511 Kev and register this energy almost simultaneously it may be assumed that the origin of the radiation is on a straight line between the two detectors. Several detectors may be used in an arrangement so that many coincident events may be imaged during the same time interval. Then the information from these detectors is processed by a computer using image reconstruction techniques in order to find the location of distribution of positron emitting isotope.
A device for imaging positron annihilation radiation requires all of the following basic parts:
(1) A number of detectors arranged in a precise geometrical pattern. These detectors are normally scintillation detectors disposed in one or several planes, and these detectors are normally arranged in a polygonal pattern on the circumference of a common circle. Scintillation detectors emit a light flash each time they absorb gamma radiation which may or may not arise from the mutual annihilation of a positron and an electron. The intensity of the light flash is proportional to the gamma ray energy. PA1 (2) The device, using scintillation detectors, must contain a means of converting the light flashes to electrical charge pulses whose amplitude is proportional to the light intensity. This may be a photomultiplier or solid state device. PA1 (3) The device must contain a means of determining that any charge pulse could have arisen from a gamma ray whose energy was approximately equivalent to the mass of the electron at rest (e.g. 511 Kev). PA1 (4) The device must have an electronic circuit capable of determining that two and only two detectors each recorded gamma rays of appropriate energy within a short time interval (coincidence resolving time). These detectors are said to have recorded a `coincident event`. PA1 (5) The device must have an electric circuit which determines which two detectors, out of the many possible combinations, recorded the so-called `coincident event.` PA1 (6) The device must have a memory in which it can record how often each pair of detectors record a `coincident event.` The memory may be part of a random access memory of a general purpose computer. PA1 (7) The device is required to use an algorithm through which the information in the memory may be transformed into an image of the distribution of positron annihilation in a cross-section surrounded by the detectors. The sequence of steps described by this algorithm may be programmed into a general purpose computer.
Accordingly, it is an object of this invention to provide a positron annihilating imaging device which has an efficiency greater than previously existing devices in its class.
Another object of the invention is to prevent gamma rays entering a given detector which are not absorbed because of the imperfect stopping power of the detector, from reaching another detector, so that all or part of the energy of the gamma rays is dissipated in a second detector.
A further object of the invention is to increase the efficiency of the individual detectors, in a positron annihilation imaging device, to radiation which is not incident normal to the detectors' face. Since any detectors may be in coincidence most of the radiation falling on the detectors is not incident normally on it but at an angle which is significantly different from 90.degree..
A further object of this invention is to enable heavy metal objects to be placed in-between the detectors, in a positron annihilation device, in such a way as to decrease the apparent widths (aperture functions) of the detectors, which, while reducing the efficiency of individual detectors, reduces their aperture functions allowing a subsequent increase in spatial resolution.
In accordance with an aspect of this invention, the detectors are arranged in planes and around the circumference of common circles.
In accordance with an aspect of this invention, the detectors may be basically trapezoidal in shape with the smaller face of the trapezoid being disposed on a circle extending around the body undergoing examination. The outer face of the trapezoid which is larger, is in optical contact with the face of an associated photomultiplier tube or other device which can convert light into an electric pulse.
In accordance with another aspect of this invention, the detectors are made from bismuth germanate, a dense scintillating crystal which is approximately 2.5 times more efficient than sodium iodide crystals.
In accordance with another aspect of this invention, the bismuth germanate detector crystals are separated by tungsten septa which decreases the probability of the penetration of gamma rays from one detector to the other.
In accordance with another aspect of this invention, the shape of the detector crystals may be further modified by cutting the corners of the front face of the detectors at an angle of 45.degree. giving the front of the detector a somewhat pointed shape.
In accordance with another aspect of this invention, between the shaped detector fronts may be inserted a heavy metal diamond shaped plug which can be removed at will. The purpose of this plug is to change the aperture function of the detectors allowing a trade-off between high efficiency and high spatial resolution.
In accordance with another object of this invention, the placement of the above-mentioned heavy metal objects and their shape is such as not to impede the intensity of incident radiation for angles up to about 30.degree. from the perpendicular to the front face.
In accordance with the foregoing aspects of the invention, there is provided:
A scintillation detector for use in the detection of annihilation radiation in a positron disintegration process, said detector comprising a right prism of bismuth germanate, one end face of said prism being in optical contact with a light-amplifying device and the other end face of said prism being exposed to said radiation, at least the side surfaces of said prism having a light reflective coating thereon.